Not a easy question to answer, my question is why not have both? besides the cost
-What is your breakdown of movies to music? What types of movies do you enjoy watching
-Why does it have to be a permanent thing? You can always build back up from two channel, then again the cost?
-Are you not happy with your current L/R Speakers? Do you really believe that McIntosh will make them sound that much better?
I upgraded my speakers because I wanted better two channel sound and was sold on GE HVFR Tweeter. I think I get excellent two channel sound along with great Home Theater. Not to knock McIntosh, but the biggest increase in sound performance you are going to get come from the speakers, emotional attachment aside.
If you aren't watching movies much, or the types of movies that really take advantage of surround sound aren't the types of movies you watch, then maybe 2 channel is the way to go. For me, I watch a lot of action movies, can't wait for Captain America Winter Soldier to come out on Blu-ray and love the smile movies like that and the Hobbit put on my face enjoying the surround effect. If that isn't you, you may have answered your own question.
At the end of the day if all you want out of 2 channel vs. surround is to own McIntosh Gear, that's really up to you, and while I don't see it as a huge advantage, if it's an emotional attachment that is important for you than go for it.
In building my new HT area, I am stealing from my bedroom setup (Triton 7s) and going strictly 2 channel. 75% of the time if I am tired I will watch TV in my bedroom, but if it is a new release blue ray, the HT is where I go.